One of the more popular optical illusions in art history is anamorphosis. It's a mind-bending form of art that dates back to the 1500s, and is still causing people to cock their heads and blink today. Take a look at the weird designs that artists come up with when they smush their own paintings.

Anamorphosis is an ancient painting technique. It may actually be the ancient painting technique, because some art historians say that it was used in the cave paintings of Lascaux. Anamorphosis is distortion of an image so that it appears in perspective only from one angle. In Lascaux, supposedly the horses and bison painted on the walls are distorted so that they seem in perfect perspective when looked at from the floor below. Michelangelo's David is sculpted so that the head and upper body is proportionately larger than the lower body, giving the illusion of a normal body from those looking up at it. But that's the invisible form of anamorphosis - the kind at work when people don't want you to notice it. It's much more fun when they do want you to see what they've done.

The most famous instance of anamorphosis is undoubtedly Hans Holbein's painting, The Ambassadors. Painted in 1533, it shows two opulently dressed men standing in a room filled with the symbols of empire. Globes, sundials, astronomical instruments, all done in Holbein's characteristic stiff and slightly flattened style. The painting seems to be marred by a white and gray smudge across the bottom. Only as person walks down and to the side does the smudge resolve itself into a three-dimensional looking skull. The painting, then, is meant to be viewed from two different angles; straight on and staring at humankind's achievements, and from down below, remembering how all people, great and small, end up. It's said that the painting was meant to hang in a stairwell, and that as people walked down the stairs, they would see the skull suddenly snap together as they passed the painting.

Another theory is Holbein was showing off that he could do more than one style of art, because anamorphic images were doing a brisk business which lasted for the next few hundred years.. Anamorphosis filled needs that regular painting and decoration simply couldn't. One of those was practical. The most famous anamorphic image in architecture is the dome of Saint Ignazio, painted originally by Andrea Pozzo. The dome itself doesn't exist. The devout brothers had built the church, but sadly the cupola they expected to build never materialized. Tired of looking up at a disappointment, the brothers hired Pozzo to paint a fake dome interior on canvas. In most areas, the dome looks strange and distorted. There is a certain spot, though, that makes the dome look like the giant, vaulting thing that had been originally envisioned by the church. The ceiling also features frescoes that make the relatively low ceiling look like a much taller arch. Today, we're more likely to see anamorphosis on the ground. If you've ever seen those chalk paintings that make it look, from a certain angle, like the sidewalk is crumbling into lava or characters are coming up out of the ground, you're seeing the modern version of the Ignazio dome.

Anamorphosis was also the 3D tech of its time. It allowed objects to be projected in such a way that they had depth that they couldn't acquire any other way. To no one's surprise, the paintings that made full use of this kind of tech were nudes. A painting would be painted of a nude with an appendage - usually a hand or a foot - painted in an anamorphic style. A reflective pillar or ball would then be put on one part of the painting, and the appendage would be reflected in it, in proper perspective. This made it look like the nude was extending a hand or foot to the viewer. Other more conventional paintings were just messes of smashed color, with a pillar to be placed in the middle. In the pillar, the painting would be reflected in perfect perspective, giving it all depth. With the right tones, the painting itself would be indecipherable until the pillar was placed, making it the secret of whoever held its accessories.

For the most part, anamorphosis is now a novelty in art. There are desk toys and children's activity books that are made to showcase it. Occasionally though, like Holbein, modern painters will showcase their style and ability to paint perspective.